In my prior U.S. Pat. No. 2,622,601 I have described an apparatus of this general type wherein currents of a medium frequency, in a range between 1,000 and 10,000 Hz., are passed over separate but intersecting paths through a body to be treated, with generation of low-frequency beats in the region of the intersection. As discussed in that prior patent, alternating currents in this medium-frequency range have no significant effect upon muscle and nerve tissues whereas frequencies below this range, especially those on the order of 100 Hz., act as stimulators. Thus, the heterodyning of two slightly different medium frequencies in the interior of the body gives rise to low frequencies having the desired therapeutic effect.
There are also instances in which a stimulation closer to the surface of the body is useful, i.e. in the vicinity of the skin-contacting electrodes between which the alternating currents are transmitted. While the medium frequencies will be ineffectual in this regard, it has already been propsed to use a carrier frequency amplitude-modulated by a low frequency for such purpose. Reference in this connection may be made to my Austrial Pat. No. 296,496 as well as Austrian Pat. Nos. 165,657, 203,147 and 332,528.
As further discussed in my prior U.S. Pat. No. 2,662,601, continued application of body-stimulating low frequencies results in an accommodation or fatigue phenomenon, i.e. a decline of the original stimulating effect. To avoid this inconvenience, it is necessary to vary the intensity of the current at a relatively slow rate, generally with a period upwards of one second. In my recently issued U.S. Pat. No. 4,023,574. I have disclosed a system which achieves this result by the interaction of three frequencies of at least 1,000 Hz, two of these frequencies differing from each other by a value of 50 to 100 Hz. whereas the third one differs from one of the first two frequencies or from their arithmetic mean by 1 Hz. or less. The system of this latter patent also has no significant therapeutic effect in the vicinity of the body-contacting electrodes.